This invention relates to low pressure refrigeration systems and, more particularly, to a method and apparatus for controlling the refrigerant gas pressure in such a system. Specifically, this invention relates to a method and apparatus whereby the refrigerant gas pressure may be quickly raised so as to avoid unwanted time delays when it is desired to leak test a low pressure refrigeration system.
A popular form of large air conditioning systems is a centrifugal chiller system wherein water is cooled and then piped to a heat exchanger for cooling circulated air. The basic components of such a system are an evaporator, a condenser, and a centrifugal compressor. The water is circulated in pipes within the evaporator and is cooled by refrigerant in the evaporator vessel. The cooled water is piped to a heat exchanger for cooling circulated air. Typically, such a system uses low pressure refrigerant, for example, R-11 or R-123. With such a refrigerant, at temperatures below approximately 75.degree. F., the refrigerant is in a relative vacuum with respect to the ambient atmosphere in which the chiller system resides. As is conventional, such chiller systems are regularly subjected to leak tests to determine the location of leaks which develop, due to vibration, during the normal operation of the refrigeration system. Such leaks are undesirable in that they allow envirormentally deleterious refrigerant to escape during the operation of the refrigeration system and allow contaminants to enter the system when it is shut down or inoperative. To perform a leak test, the refrigerant lines must be pressurized. At one time, it was common to use nitrogen or air to pressurize the refrigerant lines. However, this technique is disadvantageous because after the leak test is completed, the lines must be purged of the nitrogen or air which was introduced for the test, resulting in a loss of refrigerant, in addition to requiring a substantial amount of time. Accordingly, various methods and apparatus have been developed whereby the pressure of the refrigerant gas itself is increased. All of these methods and apparatus known to the applicant involve the heating of the refrigerant to increase the refrigerant gas pressure. Thus, U.S. Pat. No. 4,862,698 discloses a system wherein chiller water is diverted to a heater and the heated water is circulated through the evaporator to heat the refrigerant therein. U.S. Pat. No. 4,864,829 discloses a system wherein the refrigerant within the evaporator is heated. U.S. Pat. No. 5,142,876 discloses a system wherein liquid refrigerant is removed from the evaporator, heated in an external heater, and returned to the evaporator. While effective, all of these disclosed systems have a common disadvantage. This disadvantage is that in order to raise the pressure of the refrigerant gas, the liquid refrigerant and the metal evaporator shell are heated, in addition to the water in the chiller pipes within the evaporator. Typically, an evaporator shell weighs approximately 1000 pounds and contains approximately 2000 pounds of liquid refrigerant. In addition, there is approximately 1000 pounds of chill water. It can be calculated that if it is desired to raise the temperature of the refrigerant from 50.degree. F. to 80.degree. F. in order to have sufficient pressure for a leak test, this requires the application of approximately 120,000 British Thermal Units, or approximately 35 kilowatt hours of heat energy. The amount of power that can be applied is limited because heat inputs of over 6 watts per square inch can cause overheating of the refrigerant and there are only about 2500 square inches of evaporator shell surface area available. Assuming high energy transfer efficiency, it will therefore take three hours for the refrigerant to reach the desired temperature if 12,000 watts of power are applied.
It is therefore a primary object of the present invention to provide a method and apparatus whereby leak testing of a low pressure refrigeration system can be accomplished in a time effective and energy efficient manner.